I like your predictions but I don't think Luffy can go from 1.5 B to 5 B. After Wano he will be a "junior Yonko", so to speak. I can't see him almost match Whitebeard's bounty despite needing a ton of help to beat Kaido. No matter what happens now, Kaido's imminent defeat is already the result of a group effort because of what happened earlier and it's unlikely he's done getting help in this fight.
But if Oda doesn't go that route, then Luffy getting 5 billion is the next logical thing as Luffy always gets bounties higher than the bounties of the opponents he's defeated.
If that's still too high then we can take a billion off a give Luffy 4 billion, putting his bounty close to Shanks, even though Luffy probably won't fight or meet him right after beating Kaido. But 4 billion will put Luffy at proper level yonko bounty along the likes of Shanks, Big Mom and Kaido.
If 4 billion is still too high and we take off another billion, down to 3 billion i think it should be fine. I definitely think that after beating Kaido, Luffy should get a higher bounty than Teach's current 2.2 billion.